1. Field of the Invention
The present invention relates to determining traveltimes for seismic energy and more particularly to generating source and receiver traveltime tables very efficiently for complex velocity models so that depth migration can be economically performed.
2. Related Prior Art
Prior art has illustrated many ways for producing traveltime tables. Traveltime tables are usually generated by shooting a fan of rays and interpolating between the rays. There have been problems with critical angles in prior methods. There have also been difficulties in prior methods for complex velocity models wherein velocity boundaries are not layered and do not follow a natural up and down order, or where the models do not fit a simple grid image.
At present, the source and receiver traveltime tables are generated by shooting a fan of rays and traveltimes are obtained by recording the traveltimes along the ray path. The complete traveltime table is then obtained by interpolation. Prior art examples of this type of ray tracing to obtain traveltime tables is illustrated in the following United States patents.
U.S. Pat. No. 5,067,113 entitled "Efficient Generation of Traveltime Tables for Two Dimensional and Three Dimensional Prestack Depth Migration", (Hanson, Wang, Harlan) relates to a method for generating traveltime tables for application in depth migration which uses a velocity model which includes a plurality of velocities in multiple layers between various subsurface reflectors. The model is plotted on a two dimensional grid with the subsurface reflectors identified. A traveltime to the first reflector is determined. Traveltimes from the sources on each layer boundary to all grid points above the next reflector are determined. The method determines the minimum traveltime from the source to each layer boundary proceeding from the surface downward. The traveltime to each layer boundary is determined from that of the overlying layer boundary. Therefore for layers below the surface, there will be more than one source or secondary source.
U.S. Pat. No. 4,839,869 entitled "Methods for Processing Converted Wave Seismic Data", (Corcoran) relates to methods for processing converted wave seismic data which includes, fractional point gathering of the data in a manner consistent with a selected velocity model, dynamic correction of the data using parameters measured from the data to account for the asymmetric travel path of the converted wave rays and stacking the dynamically corrected data. Methods are also provided for updating the velocity model.
U.S. Pat. No. 4,964,088 entitled "Method for Tomographically Laterally Varying Seismic Data Velocity Estimation", (Chittineni) relates to a method for velocity estimation of seismic data using seismic tomography processing. The method uses traveltime inversion tomography wherein the reflectors for each source and receiver pair are picked from prestack seismic data. This is a method for velocity estimation and analysis wherein the seismic section is divided into cells of preselected sizes with each rectangular cell assigned a different velocity function based on the velocities on its four corners. Beginning with a depth model, traveltimes are computed by tracing rays, and subsequently the parameters of the model are updated by comparing the estimated traveltimes with the true traveltimes. The two sets of traveltimes are compared and the differences used to update the model velocities until convergence criterions are satisfied. The final velocity model can then be used for other processes such as depth migration.